1. Field of the Invention
The invention relates to new polymeric compositions derived from aromatic polyimides containing terminally unsaturated groups. More specifically, it relates to such polymers derived from polyimide compositions in which the terminal groups are moieties that contain terminal diacetylenic functions --C.tbd.C--C.tbd.C--, capable of polymerizing and forming crosslinked polymers. Still more specifically, it relates to such crosslinked polymers obtained therefrom without the formation of by-products.
2. State of the Prior Art
Polyimides, as prepared from aromatic dianhydrides and aromatic diamines, are known to have the desired property of high heat resistance and high solvent resistance. Such polyimides, upon condensation to an infusible condition, generate by-products such as water and other vapors or gases which introduce voids into the fabricated products that detract from the expected good physical properties. In addition, because of these same desirable properties, they are untractable and, therefore, very difficult and expensive to work into desired shapes and forms.
Recent patents, such as U.S. Pat. Nos. 3,845,018, 3,864,309, 3,879,395 and 3,998,786 are directed to improving the tractability of the aromatic polyimides by attaching various terminal groups to polyimide oligomers whereby the chains are extended by coupling of the terminal groups. In these patents the coupling groups are attached as terminal imide moieties containing vinyl, ethynyl, nitrile, etc. groups. Thus the terminal anhydride group is converted to an imide group containing a vinyl, nitrile, ethynyl, etc. group. However, in none of these patents nor in any other related prior art references, has there been found any reference or disclosure that the terminal anhydride group on each end could be converted to a terminal imide group containing a diacetylenic moiety possessing a --C.tbd.C--C.tbd.C-- structure.
Previous publications (A. L. Landis, et al, Polymer Preprints 15, 533 (1974), 15, 537 (1974)) describing acetylene-terminated polymers assumed that the cure reaction involved a simple trimerization of three terminal acetylenic end-groups into an aromatic crosslink, thus: ##STR1##
When model compounds, such as phenyl acetylene and others are subjected to "cure" conditions (Technical Report AFML-TR-76-71 June 1976, pp. 8-11, 21-22), they do not give rise to the formation of significant quantities of trimerized products as has been assumed. In addition to major amounts (90%) of polymeric material, there is isolated small quantities of complex mixture of products. It has been shown that terminated acetylene groups can simultaneously react by a number of alternate routes, such as by:
1. Glaser coupling (G. Glaser, Ann. 137, 154 (1870), (which in the presence of air is referred to as oxidative coupling): ##STR2##
2. Strauss coupling (F. Straus Ann. 342, 190 (1905) ##STR3##
3. Straus or Glaser product reaction (Chemistry of Acetylenes, H. G. Viehe, ed., Marcel Dekker, N.Y. 1969): ##STR4##
In order to determine the possibility that Straus or Glaser linkages are initially formed which then undergo further reaction and rearrangements, the model compounds ##STR5## were prepared and subjected to cure conditions (Technical Report AFML-TR-76-71 June 1976). Model compound A, the Glaser coupling, produces 100% polymers. Model compound B, the Straus coupling, also produces polymers along with small amounts of phenyl naphthalene. These and DSC analysis indicate that the crosslinking reaction of the terminal acetylene group is much more complex than originally assumed and apparently proceeds by a number of simultaneously mechanistic routes. Thus, if the polyimide contains terminal C.tbd.CH groups, and in the cure reaction there is produced Glaser or Straus couplings in the cured products, sensitivity to oxygen at high temperatures is to be expected. Thermogravimetric analysis of a polymer derived from: ##STR6## shows (Technical Report AFML-TR-75-30, July 1975, p. 61) that 75% of the sample weight is lost in the interval 560.degree..+-.90.degree. C. and that unusual behavior occurs above the region of the weight loss, indicating weight gain and that "the cause should be investigated".
Accordingly, it would appear to be a worthwhile technical objective to achieve the synthesis of polyimides which are devoid of terminal acetylenic structures, that is monoacetylene --CH.tbd.CH, but which still can undergo practical desirable acetylene-type chemistry to achieve the crosslinking of the polyimide. We have now discovered that this objective can be achieved by end-capping the polyimides with groups containing the diacetylene structure: --C.tbd.C--C.tbd.C--.